1. Field of the Invention
The invention relates to novel colorless ferric alkylphosphates which are useful in thermographic materials. At elevated temperatures these ferric alkylphosphates react with a range of colorless chelates (e.g., catechols, .beta.-diketones, o-hydroxynaphthamides, derivatives of salicylic acid) to give intense dark images. The thermographic materials are stable at room temperatures.
2. Background of the Art
For many years heat-sensitive imaging sheets have been used for copying, thermal printing, thermal recording, and thermal labeling. Many of these materials involve thermally increasing the reactivity of two or more components of a color forming reaction which do not react substantially at normal room temperatures. One commonly used reaction system involves a ligand or chelate and a transition metal salt which on thermal stimulation react to form a complex exhibiting an intense and stable color (e.g., U.S. Pat. No. 2,663,654). Transition metals used include iron, nickel, and cobalt and the salts formed are frequently of organic acids such as long-chain carboxylic acids and aromatic acids such as naphthalic acid (e.g., U.S. Pat. Nos. 3,094,620; 3,293,055; 3,953,659). Ferric iron is probably the most preferred transition metal (U.S. Pat. No. 4,531,141) because of its intense near black images and its controllable reactivity with ligands and chelates.
However, many of the ferric salts of organic acids are heavily colored in themselves and most have some objectionable level of coloration. It has been of considerable interest, as evidenced in the published art, to reduce the colored background tint in thermographic materials and sheets (U.S. Pat. Nos. 3,953,659; 4,531,141). These latter approaches include efforts to mask the inherent color tint of the ferric salt by the addition of white filler materials or by modifying the optical properties of the layer surface.
Amongst the many heavy metal salts of organic acids which have been disclosed as reacting with ligands and chelates to give a visible image, a number are colorless (U.S. Pat. No. 3,111,423; 3,293,055). The metals involved, such as zinc and aluminum, must however be reacted with ligands or chelates which contribute the color and indeed have a color cast before reaction. The reacted color is not very intense in most cases.
Organic phosphorus-iron compounds are known in the art which are substantially colorless. U.S. Pat. No. 4,049,612 discloses colorless ferric salts of organo-phosphinic acids which, however, have high melting points (greater than 300.degree. C.) and as a result have low reactivity at typical thermal imaging temperatures. In U.S. Pat. No. 3,483,143 ferric salts of organo-phosphinic and organo-phosphoric acids are disclosed as components of inorganic polymer systems with high structural strength and high melting points (greater than 400.degree. C.). Smythe et al. in J. Inorg. Nucl. Chem., vol. 30, pages 1553-1561 (1968), discuss the infrared spectra of ferric di(2-ethylhexyl)phosphate. In U.S. Pat. No. 4,533,930 ferric salts of organo-phosphoric and other phosphorus acids are disclosed as being near colorless and suitable for thermographic reactions and imaging processes. The preferred salts of that patent involve the addition of a carboxylic acid in the formation of the salt which is said to give a "composite iron salt".
The present invention identifies novel ferric di-alkylphosphates which are truly colorless, have melting points between 50.degree. C. and 300.degree. C., and react with ligands or chelates to give intense dark colors under the influence of heat.